Operation Overlord[ was the code name for the Battle of Normandy, the operation that launched the invasion of German-occupied western Europe during World War II by Allied forces. The operation commenced on 6 June 1944 with the Normandy landings (Operation Neptune, commonly known as D-Day). A 12,000-plane airborne assault preceded an amphibious assault involving almost 7,000 vessels. Nearly 160,000 troops crossed the English Channel on 6 June; more than three million troops were in France by the end of August.Allied land forces that saw combat in Normandy on D-Day itself came from Canada, the United Kingdom and the United States. Free French Forces and Poland also participated in the battle after the assault phase, and there were also minor contingents from Belgium, Greece, the Netherlands, and Norway.